The present intention relates to the field of artificial mineral wools. It is more specifically targeted at mineral wools intended to manufacture thermal and/or acoustic insulation materials or substrates for soil-less culture and in particular those more commonly denoted under the term of glass wool or rock wool.
The mineral wools concerning the invention are generally obtained by fiberizing processes of the so-called internal centrifuging type comprising, in general terms, pouring a molten mass of the vitrifiable raw materials into a centrifuge rotating at high speed, the periphery of which is pierced by a large number of orifices, and projecting the molten mass from the orifices in the form of filaments which are carried away and drawn into fibers by a gaseous stream at a high temperature. For more details regarding this type of technique, reference may advantageously be made, for example, to Patent EP-B-0,519,797.
In recent years, there has been much debate on the question of knowing whether this type of mineral wool could be harmful to the health, at least as regards the portion of fibers with a fineness which enables them to be accidentally inhaled. There has been speculation over the potentially pathogenic, in particular carcinogenic, nature of an excessively large accumulation of fibers in the body. This is the reason why a number of studies have been carried out in order to adapt the chemical composition of these fibers in order for them to exhibit biodegradable properties, in the sense that they can dissolve rapidly in physiological medium.
New glass compositions have thus been developed for the purpose of increasing as far as possible this biodegradability, while retaining the other properties desired for this type of mineral wool, in particular their ability to be fiberized by conventional fiberizing techniques, their mechanical properties and their temperature behavior, As an example, European Patent Application EP-0,412,878 describes mineral wools obtainable by internal centrifugation processes which have been shown to demonstrate solubility in physiological medium in vitro.
Studies have recently been carried out in Germany for the purpose of quantifying the biodegradability of mineral wools, which studies have resulted in a government recommendation which is based on the calculation of an index known as "KI". This recommendation specifies that, using this index, it is possible to classify mineral wools in various categories. Of particular interest is a category, (referred to as "category III"), which includes mineral wools with a KI index greater than 30 and less than 40, and a category corresponding to mineral wools with a RI index of at least 40. In the case of category III wools, a KI index of 30 to 40 constitutes a first level of harmlessness of the wools, in which it is necessary subsequently to confirm harmlessness by additional tests. In the second case, with a KI index of at least 40, it is possible to treat the mineral wools as harmless, without additional tests being necessary. This KI index is calculated using the percentages by weight of alkali metal oxides Na.sub.2 O and K.sub.2 O, of alkaline-earth metal oxides CaO, MgO and BaO, of alumina Al.sub.2 O.sub.3 and of boron oxide B.sub.2 O.sub.3 in the following way: EQU KI=.SIGMA.(Na.sub.2 O+K.sub.2 O+CaO+MgO+BaO+B.sub.2 O.sub.3 -2.times.Al.sub.2 O.sub.3
This very general formula allows a great number of compositions, depending on the amounts of the various constituents used to calculate this index, and depending on the nature and the amount of the additional constituents of the composition, in particular silica mostly, or alternatively iron, and the like.
A first group of mineral wool compositions observing a KI index value of at least 40 has been described in U.S. Pat. No. 5,523,26S and U.S. Pat. No. 5,523,264. In the compositions described the level of alumina is low and, in contrast, the level of boron oxide used is significantly high, of the order of 15%.
This choice is not without disadvantages, in particular of an economic nature. Raw materials containing boron are among the most expensive of the vitrifiable materials commonly used in formulating mineral wool compositions.
Other mineral wool compositions having a KI index value of greater than 40 have been described in Patent WO-95/32927. These compositions also contain a relatively high level of boron oxide and low levels of alkaline-earth metal oxides.
The aim of the invention is consequently the development of new mineral wool compositions, in particular of the type of that obtained by internal centrifuging, in particular glass wool compositions, which exhibit a biodegradable nature, being reflected in particular by a KI index value of greater than 30 and, preferably, of at least 40, without this property being obtained to the detriment of other advantageous properties, (in particular in terms of ability to be fiberized, temperature behavior or mechanical properties), while taking into account, and while limiting as far as possible, the manufacturing costs and in particular those related to the choice of the vitrifiable starting materials.